There have been many different types and kinds of reflow soldering systems for mounting microelectronic devices to a printed circuit board. A typical reflow soldering system generally includes a stencil printer for depositing solder alloy containing paste at points of attachment on a printed circuit board, one or more pick and place machines for locating the electronic components on the predeposited solder paste, a means of heating the assembly to reflow the solder paste, and a cleaning device for removing flux residues from the assembled printed circuit board.
With respect to such systems, the predeposited solder paste has generally included some type of flux material to not only enable the solder to wet the connecting metallic surfaces of the printed circuit board and the microelectronic device but to also enable heat to flow from one side of the connecting joint to another without intervention of an insulating air gap which can result in a defective soldered joint.
While the utilization of a flux based soldering paste has been satisfactory for some applications, the decomposition of the flux with resulting excessive amounts of ionic contaminants and other chemical agents at solder joints causes unwanted and undesired electrical shorting and product failures over time. Moreover, under repeated use of the heating means, which is generally a hot bar, accumulated flux and residues build up on the hot bar and produce a physical heat transfer barrier between the hot bar and the solder joint. Also, the residue build up on the hot bar results in uneven heating of the soldering surface making it difficult, if not impossible, to heat uniformly and to hold the leads of the microelectronic device in proper alignment with the connecting pads of the printed circuit board. Such residue on the printed circuit board and build up on the hot bar necessitate costly and time consuming cleaning operations which lessen throughput production.
Attempts have been made to overcome the problems associated with the utilization of flux based soldering pastes. For example, chlorinated fluorocarbons because of their ability to penetrate into small openings beneath microelectronic devices, as well as providing an inert atmosphere during the cleaning operations have proven to do an excellent job of cleaning surface mount assemblies by removing the flux residue.
While such a technique for cleaning flux residues has proven satisfactory recent environmental concerns over the emission of chlorofluorocarbon solvents into the atmosphere, and their subsequent role in the depletion of the ozone layer has resulted in restricted use of such solvents, with eventual phase out to protect our environment.
Other attempts have been made to overcome the aforementioned flux residue problems in an environmentally safer manner. For example, instead of using chlorofluorocarbon an alcohol base chemical composition has been utilized which includes chemical constituents which react with metal oxide to cause a low flux residue to occur. While such an alcohol base chemical composition helps residue flux residues, the composition has not proven sufficiently efficient for some applications.
Other alternate cleaning methods have also been developed but have also proven too expensive for automated, high yield production line systems. For example, one such system employed a heating system enclosed within a large oven filled with a large quantity of nitrogen gas to help prevent the formation of metal oxides during the reflow soldering process. While such a technique reduced the quantity of flux residues, the utilization of large ovens has proven to be unsatisfactory for most applications because the oven is very expensive to operate and the flux residues are not entirely eliminated.
More recently, other attempts have been made to develop fluxless soldering processes for surface mounting microelectronic devices. For example, N. Bandyopadhyay et al., in a recent article entitled "Development of a Fluxless Soldering Process for Surface Mount Technology" discloses a process whereby printed circuit boards and their microelectronic devices are attached with a solder paste under a reactive fluxing atmosphere. While such a system may be satisfactory for some applications, the utilization of a large oven with a large quantities of reactive fluxing atmosphere appears to be relatively expensive for assembly line production. Moreover, the disclosed technique still produces residues as the process still requires the use of a solder paste.
Therefore, it would be highly desirable to have a new and improved reflow soldering system and soldering technique which substantially eliminates the need to use any type of residue forming paste material. Moreover, such a system should not require the use of large expensive ovens or large quantities of reactive fluxing gases to clean the soldering area of undesirable and unwanted metal oxides and to enable solder to wet the metallic surfaces that are to be joined.